This document presents a method for identifying weak nodes and branches in electric power systems using metric projections. Metric projections are applied to the Jacobian matrix from a state estimator to calculate distances between nodes. Results show the voltages at each node decrease as reactive power is increased at one node. Metric projections identify nodes 1, 3 and 4 as weak based on their distances from other nodes in the Jacobian matrix. This technique can help utilities improve reactive power support and transmission capacity by identifying stressed areas of the system.

1. The International Journal of Engineering
And Science (IJES)
||Volu me|| 1 ||Issue|| 2 ||Pages|| 9-18 ||2012||
ISSN: 2319 – 1813 ISBN: 2319 – 1805
Metric Projections in State Estimation in Electric Power Systems
1,
Manuel Alejando López Zepeda, 2,Yoram Astudillo Baza, 3,Sergio Baruch
Barragán Gómez
Instituto Politécnico Nacional
Escuela Superior de Ingeniería Mecánica y Eléctrica
Departamento de Ingeniería Eléctrica
Av. Instituto Politécnico Nacional s/n, Unidad Profesional “Adolfo Lópe z Mateos”
Edif. 2, Col. Lindavista, Del. Gustavo A. Madero, D.F. C .P. 07738
---------------------------------------------------------Abstract-----------------------------------------------------------------
The identification of weak nodes and participation factors in branches have been analyzed with d ifferent
technical of analysis as: sensitivities, modal and of the singular min imu m value, leaving of the Jacobian matrix
of load flo ws. In this work shows up the application of metric projections for the id entification of weak nodes
and of the branches with more part icipation.
Keywords - Euclidean distance, Jacobian matrix, metric projections, metric spaces, state estimation.
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Date of Sub mission: 16, November, 2012 Date of Publication : 5, December 2012
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1. INTRODUCTION
Identifying weak nodes in electric power systems is a problem o f great interest because the electrical
system can reach up voltage instability and voltage collapse [1]-[3], if no relevant action is taken. Hence the
importance of identifying the nodes of the system before contingencies or demand growth. Identifying stress
peaks of the system to different scenarios of power system. Knowing weak nodes and branches with strong
participation can take action to improve the reactive power support, the margin of stability and capacity of
transmission lines. This work involves the application of metric pro jections [4] to the pro ximity of the min imu m
and maximu m distances from a given scenario with cutoff values thus identifying weak nodes and branches with
the strong participation. Traditionally the identification of weak nodes or branches are included in techniques
such as sensitivities analysis, modal analysis and the minimu m singular value, based on the analysis of the
Jacobian mat rix [5]-[7].The proposed technique performs the Jacobian analysis but from metric distances,
presenting a faster computer processing and identifying nodes weak and branches and participation. The
analysis of the distances in matrix form has been used to calculate the distances between cities, locating the
distances in matrix form and the comparison between arrays was performed. Where the measurement of
distances is performed based on the Euclidean norm [8]. It has also been used to identify leverage points in the
state estimation in electric power systems [4], [9], [10].
It is observed that there is a relat ionship between leverage points and sensitive nodes and branches as
they are generated by electrical parameters and topology of the electrical system, [4], [11], [12]. And their
structural characteristics are related to the parameters of electrical system (transmission lines, transformers).On
one side identifying atypical natures of suspicious points and on the other side weak node s and sensitive
branches are sought. In both cases, the distance of each point with respect to the total points are calculated in an
n-dimensional system.
2. M ETRIC SPAC ES
A metric space [13]-[16] is a pair where is a nonempty set and is a nonreal function defined
on , called distance or metric, and satisfies the following axio ms:
i. Non-negative:
ii. Identity of indiscernibles:
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2. Metric Projections in State Estimation in Electric Power Systems
iii. Symmetry:
iv. Triangle inequality :
For a given set may define more than one metric. When the metric of the space is required, we simply speak
about the metric space although we know that it really is a pair . The elements of the call point metric
space.
2 .A DISCRETE METRIC S PACE
Given a nonempty set , we define any discrete metric on by
It’s easily verified that is a metric space.
2.B THE R EAL LINE
Let , for every . The metric axio ms are true. The set of complex
numbers with the distance function is also a metric space.
2.C EUCLIDEAN DISTANCE
There are many different ways to define the distance between two points. The distance between two
points is the length of the path connecting them. In the plane, the distance between two points and
is given by the Pythagorean Theorem.
Let , the set of all of real numbers. If and are
elements of , we define the distance:
The above formula is known as the Euclidean Distance [25]-[28], it is the shortest distance between two points,
and it’s also known as the “standard” distance between two vectors. The first three metric axio ms are check and
it can be easily verified. The triangular inequality is described as
If we rep lace the earlier inequality and , therefore , and the
inequality is described as
This last inequality is derived fro m the Cauchy-Schwarz-Buniakovsky inequality (CBS)
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3. Metric Projections in State Estimation in Electric Power Systems
Indeed, using the inequality CBS we get
2.D THE S PACE
Let , the set of all the n-pairs of real nu mbers. If and are
elements of , we define the distance between and by
where p is a fixed number greater or equal to 1. The metric axio ms are true. To verify the triangle inequality
we make the same replacement, and then we show the Minkowski inequality.
[Minko wski]
For the inequality is trivial, for the proof is based on Hölder inequality, which is a generalized
version of CBS:
[Hölder]
where the numbers and satisfy the condition
To prove (8), consider the function with . Since , is an increasing
function for positive t. For those same the inverse function is defined. If we’ll chart the function ,
choosing two positive real numbers y , and marking the corresponding points in and axes, respectively,
and drawing straight parallel lines to the axes.
We’ll obtain two "triangles", limited by the lines, the axes and the curve, whose areas are
Furthermore, it is clear that meets . We write and , then
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4. Metric Projections in State Estimation in Electric Power Systems
Therefore, for any positive real and , and conjugate pair we have
Substituting in (10)
And summing over the index have Hölder inequality(8).
Now we show the Minko wski inequality. Consider the identity
Replace and add over the index
Apply to each of the sums on the right of the Hölder inequality and we consider that , we find
Div iding both sides by
We get
and fro m this it follows immediately Minko wski inequality.
If in the equation (6) we obtain the Euclidean distance.
2.E MANHATTAN DISTANCE
The Manhattan distance [17]-[19] estimate the distance to be traveled to get fro m o ne point to another as if it
were a grid map. The Manhattan distance between two points is the sum of the differences in these points. The
formula for this distance between a point and a point , it’s obtained fro m
equation (6) if :
The Manhattan distance is measured in "the streets" rather than a straight line. Instead of walking directly
fro m point A to point B, with the Manhattan distance you cannot walk through the buildings, but you walk the
streets. The Manhattan distance is also known as the distance "city-blocks" or distance "taxi-cab". It is named
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5. Metric Projections in State Estimation in Electric Power Systems
because it is the shortest distance that a car would t ravel in a city mov ing through the streets , as the Manhattan’s
streets (taking into account that in Manhattan there is only one-way streets and oblique streets and the real
streets only exist in the corners of the blocks).
3. LEAS T-SQUARES STATE ES TIMATION
The least-squares state estimator [20]-[23] for alternating current (AC) is based on a nonlinear model
measurements
where:
: measurement vector of dimension ,
: state vector of dimension n, where ,
:vector of the nonlinear function that relates the measurements with state vector,
: measurement error vector of dimension m,
: nu mber of measurements and state variables respectively.
The elements of are assumed to have mean equal to zero and the corresponding variance matrix is given by
. The optimality conditions are applied to the performance of , which is expressed by
where:
: Measurement residue.
Fro m equation (17) we’ll have to find the best estimate of the state vector of the system, wh ich it consist to
resolve the weighted least squares problem, that is, min imize the amount of residuals squared measures, whose
objective function can be rewritten as:
where is the element of the covariance matrix, . The optimality condition of first order for this
model can be written as:
where
It’s the Jacobian matrix of vector , of dimension . It's about finding the value of that satisfies the
linear equation (19). The most effective way to solve this equation is using the iterative method of Newton -
Raphson. Neglecting terms where second derivatives appear fro m , the linear system of equations to be
solved at each iteration is the following:
where:
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6. Metric Projections in State Estimation in Electric Power Systems
where:
the measurement error variance.
The variance provides the accuracy of a particular measurement. A larger variance indicates that the
corresponding measurement is not very accurate, so it is desirable to have small variance in measurements.
4.RES ULTS
Consolidating the results from the implementation of metrics in the Jacobian matrix of the state
estimator for a test system of 5 nodes [24], with the increase of reactive power. Voltage results. We present the
results of the behavior of the voltage of each of the nodes or the nodes with higher voltage abatement present for
each of the cases, for the increased inductive reactive power, until the last convergence point for each of the
cases. At the end the mos t sensitive nodes for each case are shown. Metrics projections results. We present
metrics projections results using the Jacobian matrix of the state estimator derived fro m power flows
measurements and power injections measurements. They show the results u sing the elements and
fro m the Jacobian matrix of the state. In the last part, we present the minimu m met rics projections (MMP) for
each case.
Fig. 1 shows the voltage behavior at each node as the increase of the inductive reac tive power in node 3.
Fig. 2 and 3 show the min imu m metrics projections by nodes of the elements and of the Jacobian
matrix state estimator considering the power flow measurements.
Fig. 4 and 5 show the min imu m metrics projections by nodes of the elements and of the Jacobian
matrix state estimator considering power injections measurements.
The results that provide metrics projections should be noted that these values are normalized with the base case.
For all cases voltage profile (VP) considered for cutoff values (CV) is 0.8. A ll metrics projections are obtained
fro m the Jacobian matrix of the state estimator.
4 .A VO LTAGE R ESULTS
It shows the variation of the magnitude of the voltage in the system with the increase of the inductive reactive
power at node 3.
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7. Metric Projections in State Estimation in Electric Power Systems
V vs Q
1.2
1.1
1
0.9
V
0.8
V1
V2
0.7 V3
V4
V5
0.6
0.5
-50 0 50 100 150 200 250 300
Q
Figure 1 Vo ltage variation with the increase of the inductive reactive power inductive at node 3.
4 .B METRICS PROJECTIO NS
1. We present the results of the minimu m met rics project ions by nodes of the elements of the
Jacobian matrix state estimator considering the active power flow measurements.
ACTIVE POWER FLOW MEASUREMENTS METRICS
1.1
MMP N1,N3
MMP N2
1 MMP N4,N5
CV = 0.5946
0.9
0.8
D
0.7
0.6
0.5
0.4
-50 0 50 100 150 200 250 300
Q
Figure 2 Metrics projections by nodes behavior with the
increase of the reactive power considering active power flo w measurements.
2. We present the results of the min imu m metrics project ions by nodes of the elements of the
Jacobian matrix state estimator considering the reactive power flo w measurements.
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8. Metric Projections in State Estimation in Electric Power Systems
REACTIVE POWER FLOW MEASUREMENTS METRICS
1.2
1.1
1
0.9
D
0.8
MMP N1
0.7
MMP N2
MMP N3
0.6 MMP N4
MMP N5
CV= 0.7609
0.5
-50 0 50 100 150 200 250 300
Q
Figure 3 Metrics projections by nodes behavior with the
increase of the reactive power considering reactive power flow measurements.
3. We present the results of the minimu m met rics project ions by nodes of the elements of the
Jacobian matrix state estimator considering the active power injections measurements.
ACTIVE POWER INJECTIONS MEASUREMENTS METRICS
1.1
MMP N1
MMP N2
1 MMP N3,N5
MMP N4
CV = 0.6214
0.9
0.8
D
0.7
0.6
0.5
0.4
-50 0 50 100 150 200 250 300
Q
Figure 4 Metrics projections by nodes behavior with the
increase of the reactive power considering active power injections measurements.
4. We present the results of the min imu m metrics project ions by nodes of the elements of the
Jacobian matrix state estimator considering the reactive power inject ions measurements.
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9. Metric Projections in State Estimation in Electric Power Systems
REACTIVE POWER INJECTIONS MEASUREMENTS METRICS
1.05
1
0.95
0.9 MMP N1
D MMP N2,N4
MMP N3
0.85 MMP N5
CV = 0.8016
0.8
0.75
0.7
-50 0 50 100 150 200 250 300
Q
Figure 5 Metrics projections by nodes behavior with the
increase of the reactive power considering reactive power in jections measurements.
4 .C ANALYSIS
Fig. 1 shows the voltage behavior at the five nodes of the system with the increase of the inductive
reactive power in the node 3, where the point of maximu m power transfer is 280 M VA R, beyond this value the
system does not converge and hence, the program gives incorrect estimates. In this figure we can identify nodes
3 and 4 as the nodes that have higher voltage depression, being near 0.6 PU at node 4, while node 3 is the most
affected reaching a value of 0.5706 PU.
Fig. 2 shows the behavior of the minimu m met rics projections by nodes usin g flow measurements
considering the elements of the Jacobian matrix. The CV is exceeded by all metrics p roject ions, the
minimu m metrics projections are regarding nodes 4 and 5 with a value of 0.4410 for both cases, performing
these projections in line 4-5 in both nodes.
Fig.3 shows the behavior of the minimu m metrics projections by nodes using flow measurements
considering the elements of the Jacobian mat rix. The VC is exceeded at 200 M VA R, the min imu m
metrics projections are regarding nodes 1 and 2 with a value of 0.5443, performing these projections in line 1-3
in node 1 and a value of 0.5510 in line 2-3.
Fig.4 shows the behavior of the minimu m metrics projections by nodes using power inject ions
measurements considering the elements of the Jacobian matrix. The CV is 0.6214 with a VP of 0.8, the
CV is exceeded at 250 M VA R, and the minimu m metrics projections are regarding nodes 2, 3 and 5 with a
value of 0.4886 in line 5-4 at node 2 and with a value of 0.5151 in line 5-4 at nodes 3 and 5.
Fig.5 shows the behavior of the minimu m metrics projections by nodes using power inject ions
measurements considering the elements of the Jacobian matrix. The VC is exceeded at 250 M VA R, the
minimu m metric projection is regarding node 3 with a value of 0.7203, performing these projection in line 5-4.
1. CONCLUS IONS
Metric projections have a similar behavior to voltage with the increasing of reactive power in one or
more nodes; it allows us to identify weak nodes of the system in a fast and reliable way, including the branches
involved. Because of the metric project ions are obtained from the Jacobian matrix of the state estimator, this
allo ws us to take into account all the parameters of the system when metrics are calculated. The metric
projections, as the state estimator, can be calculated in real time as the computational requirements by the
metrics are min imal and therefore their calcu lation is fast.
As metric pro jections are calculated between the rows of the Jacobian matrix of the state estimator
including compensator node, it let us analyse all the nodes in the system and they may alarm us in case of a
disturbance.
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10. Metric Projections in State Estimation in Electric Power Systems
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Biographies
Manuel Alejandro López Zepeda. He received BsC and Masters in Electrical Eng ineering fro m ESIM E-IPN
and SEPI-ESIM E-IPN, Mexico in 2002 and 2006. He’s currently a Co mputer Science professor at ESIM E-IPN.
His research interests span state estimation in electric power systems, intelligent control and neuronal networks.
Yoram Astudillo Baza. He’s graduated of the Instituto Tecnológico de Acapulco of Electro mechanical
Engineer in 2000. Master of Science in Electrical Eng ineering fro m the Escuela Superior de Ingeniería
Mecánica y Eléctrica del Instituto Politécnico Nacional (IPN). Currently a professor of mathematics at the
department of Electrical Engineering of the ESIM E Zacatenco del IPN. His research and interest are: Analysis
and Control of Electrical Power Systems, Electrical Machines, Intelligent Control, Adaptive and Robust, Power
Generation, Cogeneration.
Sergio Baruch Barragán Gómez. He received Masters in Electrical Engineering from SEPI-ESIM E- IPN,
Mexico in 2004. He is currently a electric power systems professor at Depart ment of Electrical Engineering of
ESIM E-IPN. His research interests: open software, analysis and optimizat ion of electrical power systems.
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